The most common method for reducing the flammability of wire and cable insulation and jacketing materials is the use of an organic bromine or chlorine compound along with antimony oxide. This system is very effective as a flame retardant, but such materials produce a dense black smoke when burned, and also produce hydrogen chloride or hydrogen bromide, which are both corrosive and toxic. Because of this, there has been a great deal of interest in flame retarded systems that produce lower amounts of smoke and toxic and corrosive gases when they are burned. There appear to be two main approaches that are being followed to meet this goal. The first is to eliminate halogens from the system and use instead large loadings of alumina trihydrate, another common fire retardant, or the similar filler magnesium hydroxide. The second is to develop additives that reduce the smoke and acid gas production of the halogenated systems. In addition to low smoke low toxicity these compositions must also have attractive physical properties in order to be used for wire and cable applications. These properties include hardness, abrasion resistance, environmental stability, deformation resistance, low temperature flexibility, oil resistance and good electrical properties. At present there are no low-smoke, low-toxicity, flame-retardant materials which are readily available although some new materials including metal hydrate filled polyethylene are becoming available.
Metal hydrates such as alumina trihydrate and magnesium hydroxide contain water bonded to a crystal structure with the metal atom. When heated to a sufficiently high temperature these compounds decompose and release water which subsequently varporizes. This process of decomposition and vaporization absorbs heat, thus slowing down the initial heating of the insulation material and consequently slows down the subsequent burning of the material. After this cooling effect is overwhelmed however, the presence of the metal hydrates has little effect on the subsequent process of burning. Unlike the halogenated flame retardant composition, metal hydrate compositions with non-halogenated polyolefins break down quickly into monomer units and burn relatively cleanly without a great deal of smoke production. In addition, since metal hydrates only add water to the system, they should not increase the emission of toxic or corrosive gases beyond what already would be produced by the system.
Polypropylene, which is readily available at a reasonable cost, has found many industrial uses because of its desirable physical properties, such as ease of fabrication by all conventional methods; high melting point of stereoregular, e.g., isotactic, polypropylene and compatibility with many other commercial resins, which permits a large number of blends having specific properties. Brittleness in these compositions can be reduced either by copolymerizing propylene with ethylene to form block copolymers or by blending homopolypropylene with rubbers.
Magnesium hydroxide fillers along with alumina trihydrate fillers have been used in flame retardant polypropylene compositions. Alumina trihydrate is generally more effective as a flame retardant than is magnesium hydroxide due to the greater amount of water incorporated in that filler, however, magnesium hydroxide has specific advantages, for example, better processability when incorporated into a polyolefin composition and a higher decomposition temperature than alumina trihydrate (330.degree. C. versus 230.degree. C.). This increase decomposition temperature allows a flame retardant polymer composition containing magnesium hydroxide to be processed at a higher temperature than a compound with alumina trihydrate. The higher processing temperatures allow much faster processing due to lower viscosities.
It has been found however that conventional magnesium hydroxide fillers cannot be successfully blended into rubber modified polypropylene compositions. These compositions when filled to a reasonable loading of magnesium hydroxide cannot be processed due to agglomeration of the filler particles. Accordingly, it would be desirable to provide a magnesium hydroxide filler which would not adversely affect the processability by agglomeration.